The invention relates to a process for the bipolar pulse-shaped feeding of energy into an electric low-pressure discharge as well as to the pertaining circuit for implementing the process. It is preferably used in systems and apparatuses of plasma and surface treatment technology which operate with pole changing frequencies of up to 200 kHz. A typical application is the process of reactive bipolar pulse sputtering for depositing electrically insulating thin layers on workpieces; for example, for the purpose of providing a mechanical protection, the reduction of wear or the improvement of sliding characteristics.
It is known to operate low-pressure discharges with a high frequency of 1 MHz to 100 MHz (German Patent document DE 39 42 560 A1). A disadvantage of using such high frequencies is that they present great technical difficulties in coupling the high frequency onto large surfaces (electrodes), and the coating rate is lower by a factor of from 10 to 100 than using frequencies in the kHz range.
It is known to operate electric low-pressure discharges in a bipolar manner by means of a pole change frequency of up to 100 kHz. When such discharges, for example, by means of two magnetron sources, are used for reactive sputtering, the required stability of the discharge can also be reached in this manner when electrically highly insulating layers are deposited. By cyclically changing the polarity, the discharge of surfaces is caused. The surfaces are completely or partially covered with insulating layers and are connected with the plasma when the pole changing frequency is sufficiently high (German Patent Documents DD 252 205 A1; DE 40 42 287; DE 41 06 770 A1).
From the above references, it is also known to feed energy into such discharges by means of alternating-current generators, particularly sine wave generators. This form of energy feeding has the following significant disadvantages. (1) It is very difficult or even impossible to feed the same power at both polarities because the impedance of the electric low-pressure discharge is unequal in the two pole directions. This is particularly true when, for example, the electrodes are formed by sputtering cathodes made of a different material or are covered with oxides in a different manner. As a result, the stable operation of the discharge at an operating point required for implementing the plasma process is impossible. (2) Furthermore, by means of sine wave generators, it is not possible to feed a predetermined ratio of the power for both pole directions, as required, for example, for magnetron sources with several galvanically separated electrodes.
It was suggested to control the feeding time of the energy as the operating time for the glow process for the generator during the positive and/or negative half-wave such that no energy is fed at least for the period of time of the half-wave (German Patent document P 43 24 683.4). This process for adapting the sine wave generator was tested only in the lab operation and has not yet been used. One disadvantage when feeding the discharge by means of a sine wave generator is the occurrence of arcing. It is not possible to lower the residual energy of the pertaining transformer or oscillating circuit, which is unintentionally converted in such an arcing, below a specific value.
Furthermore, a circuit is known which improves the switch-off action (German Patent document DE 41 27 505). However, its expenditures are too high in the required frequency range. The main disadvantage during the feeding of a low-pressure gas discharge by means of a sine wave generator is the considerable increase of the impedance of the gas discharge during each pole change which, on the whole, results in a faulty adaptation of the generator. In addition to a deterioration of the efficiency, this leads to high discharge voltages with the known danger of spark-overs.
It is also known to feed low-pressure gas discharges by means of bipolar rectangular pulse generators (European Patent document EP 0 534 068 A2; Thin Layers, Volume 4/1992, Pages 13-15). This permits the adaptation of the generator to the polarity-dependent different impedance. A given power ratio for both polarizations can also be adjusted by the dimensioning of the so-called "pulse-off" and "pulse-on" time. The circuit-caused equality of the pulse voltage for both polarizations is a disadvantage. Furthermore, as the result of limits of the technical parameters of the switching circuit elements, an upper frequency limit occurs for the pole change. Currently, frequencies above 50 kHz cannot be reached. Another significant disadvantage is the low value of the energy which can be fed into the plasma in an average time period. The cause is the limited current rising rate which can be observed after each pole change. Because this circuit has the characteristic of a pulsed voltage source, it, on the other hand, has advantageous self-stabilizing characteristics.
In addition, for the unipolar energy feeding in low-pressure gas discharges, in addition to devices for switching off the energy with a very low time constant when arcing occurs, processes are also known for the unipolar, pulse-shaped feeding of the energy by means of so-called chopper circuits (see German Patent documents 41 27 317, DE 37 00 633; DE 41 36 665; DE 42 02 425; DE 41 27 504; DE 42 39 218 A1; DE 42 30 779 Al) Partly, the periodic short-circuiting of the gas discharge is suggested for the purpose of discharging surfaces which are in contact with the plasma. However, all these solutions are not suitable for the bipolar feeding of the energy in accordance with the invention.
A device for generating low-pressure plasmas is also known into which the energy is fed in a bipolar pulse-shaped manner. The pole changing frequency amounts to less than 200 kHz. The device consists of a controllable direct current (DC) power supply with a bridge circuit. The output of the bridge circuit is connected with a current detection circuit (German Patent document DE-G 91 09 503 U1). This device operates by means of only one voltage source independently of the number of electrodes making up the device. This device has the disadvantage that it is not possible by means of it to feed the same powers in the case of each polarization if the impedances of the gas discharges are considerably different.
The invention is based on the object of providing an improved process and a pertaining circuit for the bipolar pulse-shaped feeding of energy with pole change frequencies of up to 200 kHz in low-pressure discharges. In an average time period, a power which is as high as possible is to be fed into the low-pressure discharge. The same power is to be fed into each polarization although the impedance of the gas discharge differs considerably in the two polarizations.
According to the invention, the object is achieved by a circuit for the bipolar pulse-shaped feeding of energy into low pressure plasmas with a pole changing frequency of up to 200 kHz for systems of the plasma and surface treatment technology having at least two electrodes (E.sub.1 to E.sub.n) which are in contact with the plasma, a current supply, an inductance (L.sub.1 to L.sub.n) and at least one switch (S.sub.1 to S.sub.n). One group of outputs of the same polarity of at least two potential-free direct current supplies, consisting of direct voltage sources (DC.sub.1 to DC.sub.n) and inductances (L.sub.1 to L.sub.n) connected in series with the latter, are electrically conductively connected with a same number of electrodes (E.sub.1 to E.sub.n) of a plasma device with a low pressure discharge. The respective other outputs of the direct current supplies are connected with one another. Between the resulting common pole and the connection lines of the electrodes (E.sub.1 to E.sub.n) with the one group of outputs of the direct current supplies, one switch (S.sub.1 to S.sub.n) respectively is arranged which has a common timing generator (T) for the pole change and a common trigger (A) for the switching in the case of the occurrence of an arcing. The inductances (L.sub.1 to L.sub.n) are dimensioned such that the entire energy supplied by the respective assigned direct voltage source (DC.sub.1 to DC.sub.n) can be taken up in the time interval between two pole changes. Additional advantageous further developments are also described herein. The circuit for implementing the process is further described.
A decisive advantage of the process according to the invention consists of the characteristic of the unexpectedly high current rising rate after each pole change coupled with a stable defined line feeding in each pole direction. It is obviously based on the effect of the energy accumulators divided in the form of the inductances L.sub.1 . . . L.sub.n and their influence on the dynamic behavior of the plasma impedance after each pole change. This effect is disclosed in a comparison of the process according to the invention with a process which is obvious to a person skilled in the art according to the state of the art and which uses the circuit illustrated in FIG. 1 (German Patent document DE 35 38 494 A1) . In this circuit, a direct voltage source DC is connected by way of an inductance L with a switch S. The switch S acts as a pole changing circuit which is known per se and to which the electrodes E.sub.1 and E.sub.2 of a low-pressure discharge are connected. This circuit has the character of a bipolar pulsed current source. The high current rising rate which can be achieved by means of it is very advantageous. However, it does not provide a possibility for the controlled power feeding in the case of a polarization-dependent different impedance of the discharge. The effect that the polarization-dependent differences of the impedance are critically increased which is observed, for example, in the case of the bipolar reactive sputtering of aluminum oxide, is particularly critical.
In contrast, the process according to the invention represents a pulse-shaped feeding of energy with a characteristic which, viewed over one pulse period, is that of a current source and thus permits a very high current rising rate. However, when viewed for the duration of several pulse periods, it acts as a voltage source and therefore has self-stabilizing characteristics in the above-described sense. By means of this process, the power for each polarization can be adjusted, even at a different impedance of the low-pressure discharge of the two polarizations to a given value in that the power of each of the direct voltage sources is given independently of the other.
Advantageously, in the case of the process according to the invention, the switching operations on the switches are in each case triggered simultaneously. However, it may also be expedient to close the switches for a fraction of the pulse length and thus short-circuit the low-pressure discharge. In addition, this switching condition exists during the occurrence of an arcing if a pole change is not carried out immediately. However, the simultaneous opening of the switches is not expedient.
When, for reasons of carrying out the process, a pulse length is set which is unequal for the two polarizations, that is, an unequal opening duration of the switches, the dimensioning of the direct voltage sources must take this condition into account. When two direct voltage sources are used, the voltage of one is increased by as much as the other is decreased.
A particularly precise, polarization-dependent feeding of the power into the low-pressure discharge is achieved when the power for the two polarizations is measured separately and is utilized as an actual value of at least one controlled system.
The dimensioning of the inductances must ensure that the entire energy of the assigned direct voltage source supplied in the time between two pulse changes is taken up. However, it is expedient to dimension the inductances larger in order to reduce the energy load of the switches. The dimensioning of the inductances also influences the time sequence of the current fed into the low pressure discharge. As a result, the form of the resulting current curve can be adapted to the requirements of the plasma process or the surface treatment process.
The invention will be explained in detail on several embodiments.